Additional points 373.3 Additional points
Stress is important for many aspects of rock mechanics and is relevant
to many of the chapter subjects in the book. In particular, further stress
questions are included in Chapters 19 and 20.
We re-emphasize the point that principal stress planes are defined
when new rock surfaces are created, e.g. by excavation of a slope,
borehole, tunnel or cavern. By Newton’s 3rd law -to every action there
is an equal and opposite reaction - there can be no normal or shear
stresses acting on such rock surfaces (neglecting atmospheric pressure).
Not only is the rock surface a principal stress plane, but the principal
stress acting on that plane is zero.
Thus, the process of creating a new rock surface causes the principal
stresses in the rock mass to be locally oriented perpendicular and
parallel to the surface. The principal stress perpendicular to the surface
is zero and the maximum and minimum stress values occur in the rock
in a direction parallel to the rock surface, i.e. the other two principal
stresses. It is much easier to understand underground deformations and
excavation-induced fracturing if this is borne in mind.
Remember that, although compressive stresses are usually reckoned
as positive in rock mechanics, computer programs for numerical analysis
are often developed from structural engineering codes in which tensile
stresses are positive. Always check the sign convention.